Fast Fourier Transform (FFT) Algorithms in DSP

Fast Fourier Transform (FFT)
Algorithms

Relation to the z-transform
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The DFS X(k) represents N evenly spaced samples of the z-
transform X(z) around the unit circle.

Relation to the DTFT
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and
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k

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
The DFS is obtained by evenly sampling the DTFT at w1 intervals.
The interval w1 is the sampling interval in the frequency domain. It is called
frequency resolution because it tells us how close are the frequency samples.

Sampling and construction in
the z-domain
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

DFS & z-
transform
IDFS

Comments
• When we sample X(z) on the unit circle, we
obtain a periodic sequence in the time
domain.
• This sequence is a linear combination of
the original x(n) and its infinite replicas,
each shifted by multiples of N or –N.
• If x(n)=0 for n<0 and n>=N, then there will
be no overlap or aliasing in the time
domain.

Comments


 




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else
N
n
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R
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for
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~
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(
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(
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(
1
0
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RN(n) is called a rectangular window of length N.
THEOREM1: Frequency Sampling
If x(n) is time-limited (finite duration) to [0,N-1], then N
samples of X(z) on the unit circle determine X(z) for all z.

Reconstruction Formula
Let x(n) be time-limited to [0,N-1]. Then from Theorem 1 we should be able to
recover the z-transform X(z) using its samples X~(k).
 
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Examples 5.6 5.7 Comments
• Zero-padding is an operation in which more zeros are
appended to the original sequence. The resulting longer
DFT provides closely spaced samples of the discrete-
times Fourier transform of the original sequence.
• The zero-padding gives us a high-density spectrum and
provides a better displayed version for plotting. But it
does not give us a high-resolution spectrum because no
new information is added to the signal; only additional
zeros are added in the data.
• To get high-resolution spectrum, one has to obtain more
data from the experiment or observations.

breadth-first, but with blocks of size = cache
Traditional cache solution:
Blocking
Size 8 DFT
Size 4 DFT Size 4 DFT
Size 2 DFT Size 2 DFT Size 2 DFT Size 2 DFT
p = 2 (radix 2)
…requires program specialized for cache size

Recursive Divide & Conquer is Good
Size 8 DFT
Size 4 DFT Size 4 DFT
Size 2 DFT Size 2 DFT Size 2 DFT Size 2 DFT
p = 2 (radix 2)
eventually small enough to fit in cache
…no matter what size the cache is
(depth-first traversal) [Singleton, 1967]

Goal of an Efficient computation
• The total number of computations should be
linear rather than quadratic with respect to N.
• Most of the computations can be eliminated
using the symmetry and periodicity properties
CN=N×log2N
If N=2^10, CN=will reduce to 1/100 times.
kn
N
N
kn
N
n
N
k
N
N
n
k
N
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W
W
W
W
W
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Decimation-in-time: DIT-FFT, decimation-in-frequency: DIF-FFT

Efficient Computation of Discrete
Fourier Transform
• Electrical sciences is full of signal
processing
• Digital computers paved way for reliable
signal processing
• DFT plays an important role and needs
efficient procedure for computation of
X(k),

Direct Computation of the DFT
• To indicate the importance of efficient
computation schemes, it is instructive to
consider the direct evaluation of the
DFT equation, Eq.(2.1). Since x(n) may
be complex, we can write

• From Eq.(2.2) it is clear that for each value of
k, the direct computation of X(k) requires 4N
real multiplications and (4N-2) real additions.
• Total 4N2 real multiplications and N(4N-2) real
additions.
• The amount of time required for computation
becomes large.

• Most approaches of improving the
efficiency of the computation of the DFT
exploit one or both of the following
special properties of the quantity

• Cooley and Tukey published an
algorithm for computation of DFT that is
applicable when N is a composite
number.
• Number of such computational
algorithms are known as fast Fourier
transform, or simply FFT algorithms.

Radix-2 FFT Algorithms
• Let N=2v; then we choose M=2 and L=N/2 and divide x(n)
into two N/2-point sequence.
• This procedure can be repeated again and again. At each
stage the sequences are decimated and the smaller DFTs
combined. This decimation ands after v stages when we
have N one-point sequences, which are also one-point
DFTs.
• The resulting procedure is called the decimation-in-time
FFT (DIF-FFT) algorithm, for which the total number of
complex multiplications is: CN=Nv= N*log2N;
• using additional symmetries: CN=Nv= N/2*log2N
• Signal flowgraph in Figure 5.19

Decimation-in-frequency FFT
• In an alternate approach we choose L=2, M=N/2
and follow the steps in (5.49).
• We can get the decimation-frequency FFT (DIF-
FFT) algorithm.
• Its signal flowgraph is a transposed structure of
the DIT-FFT structure.
• Its computational complexity is also equal to
CN=Nv= N/2*log2N

Radix-2 FFT Algorithms
• To achieve the dramatic increase in
efficiency, it is necessary to decompose
the DFT computation into successively
smaller DFT computations. In this process
we exploit both the symmetry and the
periodicity of the complex exponential
.

Decimation-in-time FFT Algorithm
• The principle of decimation-in-time is most
conveniently illustrated by considering the
special case of N an integer power of 2;
i.e.
• Since N is an even integer, we can consider
computing X(k) by separating x(n) into two
N/2-point sequences consisting of the even-
numbered points in x(n) and the odd-
numbered points in x(n). With X(k) given by

• Separating x(n) into even-numbered
and odd-numbered points, we obtain
or with the substitution of variables n=2r
for an even and n=2r+1 for odd ,

But
Consequently, Eq. (2.4) can be written as
Each of the sums in Eq.(2.5) is recognized as an N/2-
point DFT. Each of sums need only be computed for
k between 0 and N/2. Since G(k) and H(k) are each
periodic in k with period N/2.

The computational flow or the signal flow in computing X(k)
according to Eq. (2.5) for an 8-point sequence, i.e. N=8 is shown in
Figure below.
.

• Equation (2.5) corresponds to breaking
the original N-point DFT computation
into two N/2-point DFT computations.
Each of the N/2-point DFT computations
can be further broken into two N/4-point
DFTs. Thus G(k) and H(k) in Eq.(2.5)
would be computed as indicated next.

• Similarly,
where g(r)=x(2r) and h(r)=h(2r+1).

If the 4-point DFTs in Figure (2.1) are computed
according to Eqs. (2.6) and (2.7), then that computation
would be carried out as indicated in Figure (2.2).

Inserting the computation indicated in Figure (2.2) into the flow
graph of Figure (2.1), we obtain the complete flow graph of Figure
(2.3). We have used the fact that N/2 = WN
2.

In-place Computations
• In view of Figure (2.4), the Figure (2.3) gives the
complete computational flow graph for the N-
point computation of DFT of N-point sequence,
for N=8.
• An interesting by-product of this derivation is
that, this flow graph, in addition to describing
efficient procedure for computing the DFT, also
suggests a useful way of storing the original data
and storing the results of the computation in the
intermediate arrays.

For N=8, N/4-point DFT becomes 2-point DFT. The 2-
point DFT of, for example, x(0) and x(4) is depicted in
Figure (2.4).

We shall denote the sequence of complex numbers resulting from
the mth stage of computation as Xm(l), where l=0,1,..,N-1, and
m=1,2,.., forming an input to the (m+1)st stage and producing an
output Xm+1(l) as the output from the (m+1)st stage of computations,
it can be seen that the basic computation in flow graph of Figure
(2.3)

The equations represented by this flow graph are
Because of the appearance of the flow graph of Figure
(2.5), this computation is referred as a butterfly
computation.
Equations (2.8) suggest a means of reducing the
number of complex multiplications by a factor of 2. To
see this we note that

So the equations (2.8) become
Equations (2.9) are represented in the flow graph of
Figure (2.6).

Combining the observations in Figures (2.6), (2.5), (2.4) and
(2.3), the efficient FFT algorithm in the computational flow graph
representation for N=8 is obtained as shown in Figure (2.7). The
algorithm requires N/2log2N complex multiplications and N log2
N complex additions.

Decimation-in-Frequency FFT
Algorithm
• The decimation-in-time FFT algorithms were all
based upon the decomposition of the DFT
computation by forming smaller and smaller
subsequences.
• Alternatively decimation-in-frequency FFT algorithms
are all based upon decomposition of the DFT
computation over X(k). For N, a power of 2 i.e.
we divide the input sequence into first half and the
last half of points so that

• Separating k-even and k-odd, i.e. k=2r and k=2r+1, representing the
even-numbered points and the odd-numbered points, respectively,
so that

Thus on the basis of Equations (2.11) and (2.12) with
and
The DFT can be computed by first forming the sequences g(n)
and h(n), then computing h(n)WN
n, and finally computing the N/2-
point DFTs of these two sequences to obtain the even-numbered
output points and odd-numbered output points, respectively.

The procedure suggested by Eqs. (2.11) and (2.12) is
illustrated through signal flow graph for the case of 8-
point DFT in Figure (2.8).

Proceeding in a manner similar to that followed in deriving the
decimation-in-time algorithm, the final signal flow graph for
computation is shown in Figure (2.9).

• By counting the arithmetic operations in Figure (2.9), and
generalizing, we see that the computation of Figure (2.9)
requires N/2log2N complex multiplications and Nlog2N
complex additions. Thus the total computation is the
same for decimation-in-frequency and decimation-in-time
algorithms.
• Similar to decimation-in-time algorithm the computational
flow graph shown in Figure (2.9) will indicate the in-place
computation capability of decimation-in-frequency
algorithm.
• Figure (2.9) is the transpose of Figure (2.7).

Decimation-In-Time FFT Algorithms
• Makes use of both symmetry and periodicity
• Consider special case of N an integer power of 2
• Separate x[n] into two sequence of length N/2
– Even indexed samples in the first sequence
– Odd indexed samples in the other sequence
• Substitute variables n=2r for n even and n=2r+1 for odd
• G[k] and H[k] are the N/2-point DFT’s of each subsequence
       
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odd
n
kn
N
/
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j
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N
even
n
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j
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j
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n
[
x
e
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n
[
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n
[
x
k
X
   
   
k
H
W
k
G
W
]
1
r
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[
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N
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Decimation In Time
• 8-point DFT example using
decimation-in-time
• Two N/2-point DFTs
– 2(N/2)2 complex multiplications
– 2(N/2)2 complex additions
• Combining the DFT outputs
– N complex multiplications
– N complex additions
• Total complexity
– N2/2+N complex multiplications
– N2/2+N complex additions
– More efficient than direct DFT
• Repeat same process
– Divide N/2-point DFTs into
– Two N/4-point DFTs
– Combine outputs

Decimation In Time Cont’d
• After two steps of decimation in time
• Repeat until we’re left with two-point DFT’s

Decimation-In-Time FFT Algorithm
• Final flow graph for 8-point decimation in time
• Complexity:
– Nlog2N complex multiplications and additions

Butterfly Computation
• Flow graph constitutes of butterflies
• We can implement each butterfly with one multiplication
• Final complexity for decimation-in-time FFT
– (N/2)log2N complex multiplications and additions

In-Place Computation
• Decimation-in-time flow graphs require two sets of registers
– Input and output for each stage
• Note the arrangement of the input indices
– Bit reversed indexing
       
       
       
       
       
       
       
       
111
x
111
X
7
x
7
X
011
x
110
X
3
x
6
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101
x
101
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5
x
5
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001
x
100
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110
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011
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3
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010
x
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100
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001
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1
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000
x
000
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X
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

Decimation-In-Frequency FFT
Algorithm
• The DFT equation
• Split the DFT equation into even and odd frequency indexes
• Substitute variables to get
• Similarly for odd-numbered frequencies
  



1
N
0
n
nk
N
W
]
n
[
x
k
X
  
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1
N
2
/
N
n
r
2
n
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1
2
/
N
0
n
r
2
n
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1
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2
n
N W
]
n
[
x
W
]
n
[
x
W
]
n
[
x
r
2
X
   
 
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


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
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1
2
/
N
0
n
nr
2
/
N
1
2
/
N
0
n
r
2
2
/
N
n
N
1
2
/
N
0
n
r
2
n
N W
]
2
/
N
n
[
x
]
n
[
x
W
]
2
/
N
n
[
x
W
]
n
[
x
r
2
X
     








1
2
/
N
0
n
1
r
2
n
2
/
N
W
]
2
/
N
n
[
x
]
n
[
x
1
r
2
X

Decimation-In-Frequency FFT Algorithm
• Final flow graph for 8-point decimation in frequency

FFT vs. DFT
• The FFT is simply an algorithm for efficiently calculating the
DFT
• Computational efficiency of an N-Point FFT:
– DFT: N2 Complex Multiplications
– FFT: (N/2) log2(N) Complex Multiplications
N DFT Multiplications FFT Multiplications FFT Efficiency
256 65,536 1,024 64 : 1
512 262,144 2,304 114 : 1
1,024 1,048,576 5,120 205 : 1
2,048 4,194,304 11,264 372 : 1
4,096 16,777,216 24,576 683 : 1

Bit Reversal
The bit reversal algorithm used to perform the re-ordering of signals.
The decimal index, n, is converted to its binary equivalent.
The binary bits are then placed in reverse order, and converted back to a
decimal number.
Bit reversing is often performed in DSP hardware in the data address
generator (DAG).

DIT FFT
 Input signal must be properly
re-ordered using a bit reversal
algorithm
 In-place computation
 Number of stages: log2 N
 Stage 1: all the twiddle factors
are 1
 Last Stage: the twiddle factors
are in sequential order
Stage
1
Stage
2
Stage
3
Stage
Log2N
Number of
Groups
N/2 N/4 N/8 1
Butterflies
per Group
1 2 4 N/2
Dual-Node
Spacing
1 2 4 N/2
Twiddle
Factor
Exponents
(N/2)k
, k=0
(N/4)k
,
k=0,1
(N/8)k
,
k=0,1,
2,3
k,
k=0 to
N/2–1

DIF FFT
 Output signal must be
properly re-ordered using a bit
reversal algorithm
 In-place computation
 Number of stages: log2 N
 Stage 1: the twiddle factors
are in sequential order
 Last Stage: all the twiddle
factors are 1
Stage
1
Stage
2
Stage
3
Stage
Log2N
Number of
Groups
1 2 4 N/2
Butterflies
per Group
N/2 N/4 N/8 1
Dual-Node
Spacing
N/2 N/4 N/8 1
Twiddle
Factor
Exponents
n,
n=0 to
N/2 - 1
2n,
n=0 to
N/4 - 1
4n,
n=0 to
N/8 - 1
(N/2)n,
n=0

Radix-4 Decimation-In-Time FFT
Algorithm
A radix-4 FFT combines two stages of a radix-2
FFT into one, so that half as many stages are
required.
The radix-4 butterfly is consequently larger and
more complicated than a radix-2 butterfly.
N/4 butterflies are used in each of (log2N)/2
stages, which is one quarter the number of
butterflies in a radix-2 FFT.
Addressing of data and twiddle factors is more
complex, a radix-4 FFT requires fewer
calculations than a radix-2 FFT.
It can compute a radix-4 FFT significantly faster
than a radix-2 FFT

Inverse Discrete Fourier
Transform (IDFT)
The inverse discrete Fourier transform (IDFT) is given by
which is structurally similar to DFT,
The change we notice is in the multiplication factor 1/N} and
replacement of W N
kn by WN
-kn, and the interchange of signals
x(n) and X(k) in the expressions and the index for summation.

• Thus in Figure (2.7) and (2.9), if we exercise
the above changes, the changed signal flow
graphs will become algorithms for IDFT and
referred as IFFT algorithms.

Example
• Using decimation-in-time FFT algorithm compute
DFT of the sequence
{-1 –1 –1 –1 1 1 1 1}
• Solution: Twiddle factors are

Solution and signal flow graph of the example

Fast Fourier Transform (FFT) Algorithms in DSP

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